Tall oil is a by-product of kraft pulping of coniferous trees. It comprises resins acids, fatty acids and unsaponifiables. Tall oil is recovered from black liquor that is formed as a result of pulping coniferous trees in alkaline cooking liquor form black liquor. The black liquor contains cooking chemicals, lignin and other substances separated from the wood. The black liquor is then concentrated and left to settle. Tall oil soap rises on the surface of the black liquor and can be separated from it. The soap is acidulated by adding acid, such as sulphuric acid to the soap and crude tall oil is produced. The crude tall oil (CTO) may further be distilled for producing distilled tall oil (DTO).
CTO can be used for different purposes, for example for preparing soap, pine soap, cosmetic products or carbonaceous fuel components. The carbonaceous fuel components are produced by catalytic hydrodeoxygenation (HDO) and/or hydroisomerization (HI) of CTO, as is described for example in document WO2008/058684. One problem when using CTO as feedstock for fuel components is that metals in the CTO tend to deactivate the catalysts used in these catalytic processes. The metals in the CTO poison the catalysts active sites thus preventing the catalysts from functioning properly, Furthermore, the lignin residues in the CTO clogs the pores of the catalyst also deactivating the catalyst. Consequently, when preparing fuel components from CTO, it is purified before the HDO and/or HI steps to remove impurities, such as metals, residue lignin and/or carbohydrate derived material and possible other substances.
Document WO2008/058664 describes a process in which feedstock, vegetable oil or animal oil, is treated before HDO step to remove alkaline metals and alkaline earth metals from the feedstock. This treatment is conducted by washing the feedstock with slightly acidic media or absorbing the metals on a suitable material, for example using ion exchange resins.
Document EP 1741768 describes a process in which the feedstock, bio oil or fat, is degummed before HDO step. In degumming phosphorous compounds are removed from the feedstock and it is performed by washing the feedstock with H3PO4, NaOH and soft water and separating the formed gums. Most of the metal components in the feedstock are removed during degumming.
After the metal components removal, lignin and possible other residues are separated from the feedstock by e.g. decanting.
One of the problems associated with the prior art vegetable oil based feedstock purification methods is that they are slow and inefficient thus making them difficult to be used in processes for producing biofuels. If the feedstock is CTO, it comprises metals such as copper, iron, sodium, phosphorus, silicon and also some aluminium, calcium, potassium and lead. Washing based methods for removing metals do not decrease the metal content of the feedstock sufficiently to prevent deactivation of the catalytic agent used in the HDO and HI steps. Furthermore a considerable amount of feedstock, CTO, is lost together with the separated lignin when decanting the feedstock for lignin and possible other residues. This decreases the yield of the process such that less biofuel is obtained from a specific volume of feedstock.